This application relates to chirp spread-spectrum communication systems and in particular to chirp spread-spectrum communication systems for use on noisy network media such as power lines where signal energy must be tightly contained within a frequency band.
Spread-spectrum communication is a method whereby information is communicated using a bandwidth that greatly exceeds that required by information theory. These methods provide signals over a wide bandwidth, and with proper signal processing the communication is immune to large amounts of noise within that bandwidth. In chirp spread-spectrum methods, a signal burst known as a chirp is transmitted. Each chirp has energy spread across a frequency range. The frequency spread may be achieved by frequency sweeping or by other known techniques (such as direct sequence). Chirps may be sent asynchronously, or at synchronous intervals, including as concatenated chirps. Data modulation of the chirp stream can be accomplished by means such as phase reversal modulation of the chirps or reversal of the frequency sequence of the chirp. A transversal filter in the receiver is matched to the chirp(s) expected, enabling individual chirps to be detected even on noisy network media such as power lines.
In some applications of this communications method, it is necessary or desirable that the energy of the signal beyond the nominal signalling bandwidth be kept to a very low level. For example, conducted signal energy on the power line must be below 1000 uV in the frequency range of 535 Khz to 1710 Khz to meet FCC part 15 requirements for carrier current systems. If a wideband chirp signal of substantial energy is desired, say from 100 Khz to 400 Khz, the out-of-band energy must diminish very rapidly to meet these requirements. The alternative is to limit the transmitted signal strength, which inhibits performance. Conventional spread spectrum signalling techniques cannot normally accomplish this without the addition of costly filtering.
Another example where tightly contained signal energy is important is when adjacent wideband signalling bands are to be used. For example, it may be desirable to have a wideband chirp signal having a frequency span of 20 Khz to 80 Khz used on the same power line as a 100 Khz to 400 Khz wideband chirp signal. The simultaneous use of adjacent bands is feasible if the out-of-band energy of each is highly constrained so as not to limit the dynamic range of the other. The same chirp signal format can be used for both bands by simply slowing the 100 Khz to 400 Khz chirp signal generation source by a factor of five. In this instance, each of the signals has the same information theory processing gain, but the lower band (20 Khz to 80 Khz) has a signal power density five times that of the higher band (100 Khz to 400 Khz) and a data rate that is one fifth the higher band.
In both examples given, it is more important that the high frequency portion of the band be the most tightly constrained. In the first case, the FCC part 15 limits are effective at the higher end of the 100 Khz to 400 Khz band, and in the second case, the signal energy is five times greater in the lower band than in the adjacent upper band, allowing the higher band to have more of its lower end signal energy leak into the high end of the lower band and more tightly constraining the higher end of the lower band limits of its signal energy leaking into the lower end of the higher band to have commensurate performance.
In my previous patent entitled SPREAD-SPECTRUM COMMUNICATIONS SYSTEM FOR NETWORKS, U.S. Pat. No. 5,090,024, herein incorporated by reference, a system is proposed for communicating over power lines and the like that employs chirp spread-spectrum communication techniques.